Pad-type printers are commonly used for printing text or images onto three-dimensional articles. Typically, a pad type printer includes a horizontal reciprocating printing plate having an image engraved onto the surface of the printing plate. The engraved image is commonly supplied with ink from an open reservoir or trough by means of a brush, spreader blade, roller or wire applicator. A doctor blade is then used to remove excess ink from the printing plate so that the ink remains only in the recessed regions of the engraved image. Finally, a rubber printing pad transfers the ink from the engraved image to a desired article.
A problem with pad-type printers having open ink reservoirs is that the ink has a tendency to evaporate from the open reservoir. Ink evaporation is undesirable in that the viscosity of the ink changes as the ink evaporates. Additionally, the evaporating ink causes vapors, creating a health concern. In order to reduce ink evaporation, many current pad-type printers employ closed ink cups. By reducing ink evaporation, vapors are reduced and the viscosity of the ink is more consistent, thereby increasing ink life.
A closed ink cup is usually a cup shaped member which is inverted over the printing plate so that the printing plate holds the ink within the ink cup. The rim of the ink cup contacts the printing plate and serves as a doctoring edge. Ink is supplied to the engraved image when the engraved image is translated beneath the ink cup. The doctoring edge wipes excess ink from the printing plate while leaving sufficient ink on the engraved image.
One common type of closed ink cup is a one-piece cup machined from a single block of steel. One-piece ink cups are precision machined in a lathe or a computer controlled milling machine and are then heat-treated to harden the doctoring edge. The doctoring edge is then lapped to provide a flat and consistent surface. Problems with one-piece ink cups are that they are easily damaged because they are brittle and they are expensive to manufacture. If the doctoring edge of a one-piece ink cup is damaged (for example, scratched, chipped or cracked), the ink cup must be replaced or repaired because a damaged doctoring edge will cause the ink cup to leak. Additionally, the use of a one-piece ink cup usually requires the use of an expensive precision ground printing plate. A one-piece ink cup is rigid and is not capable of satisfactorily wiping ink from the surface of an inexpensive non-precision ground printing plate because the doctoring edge cannot flex to follow the ripples and craters on the surface of the printing plate.
The replacement of an entire ink cup can be very expensive. U.S. Pat. No. 5,320,037 proposes a multiple part ink cup having a replaceable sheet metal band which forms the doctoring edge. As a result, only the doctoring edge requires replacement if damaged. However, the large number of interlocking parts forming this ink cup makes it both difficult to clean and expensive.